Express.js is the workhorse of the Cloudcraft.co backend. It's not the most exciting part of a stack, but it works, is very well documented, and you can find a plugin for almost everything you could possibly want. We also carefully evaluated Koa.js, but decided not to go down this route: fewer plugins, less documentation & answers online. I'm also not personally convinced by the generators yield syntax at all. ES7 async functions looks like a much better bet, and with Promises and Babel I can have that already today.

I'm fluent in ExpressJS, but over the past two years I have moved to HapiJS. Similar results, but I find Hapi to be more full-featured towards my app, api and service needs. I can operate confidently in both.